1. Field of the Invention
The present invention relates to special metallurgy, and, in particular, to plasma metallurgy, and is particularly concerned with the design of a plasma-arc furnace.
The invention may prove most advantageous in the manufacture of plasma-arc furnaces provided with ceramic lining, furnaces having slag lining, furnaces provided with a water-cooled mould etc. It may be utilized in all the cases where it is necessary to carry out melting and remelting metals, heating and fusing ingots, rolled stock, forgings and other metallic blanks, said operations being carried out either in air or in a controlled atmosphere to eliminate defects of the surface layer thereof.
2. Description of the Prior Art
Plasma-arc furnaces provided either with DC or AC plasma torches are used at present for melting, remelting, heating and fusing metals. Furnaces having AC plasma torches are most generally employed since the use of such torches, as compared with DC torches ensures numerous advantages, the main among them being the following:
absence of magnetic interaction between plasma arcs of adjacent torches, said interaction resulting in a decrease in the furnace efficiency and in damaging the torches;
lower cost and simplicity of power supplies;
low tendency to double arc formation which hereinafter means transfer of a plasma arc to the torch nozzle, due to which fact the arc burns not only between the torch electrode and the metal being heated, but also between the electrode and the nozzle, thereby resulting in a decrease in the service life of the latter, and in a drop in the furnace efficiency.
Alongside with the above advantages, furnaces provided with AC plasma torches, especially single-phase torches, feature some disadvantages, among them first of all a low stability of burning of plasma arcs. In such furnaces, a single-phase AC power supply is connected to an electrode of the plasma torch and to a hearth electrode being in contact with the metal being heated. Instability of arc burning between electrodes is expressed in burning interruptions caused by repeated changing in electrode polarity in the course of the AC current flow. This fact results in significant difficulties when carrying out numerous metallurgical processes where it is necessary to ensure reliable arc-striking during each half-cycle of the current flow with a significant (in the order of 1 m) arc length.
Plasma arc length is further used to mean a magnitude corresponding to a distance from the working end of the plasma torch electrode to a surface being treated.
Known in the art are plasma-arc furnaces provided with single-phase AC torches, wherein in order to improve the stability of arc burning are applied additional high-voltage power supplies generating high-frequency electric oscillations which are superposed on the AC current of the main power supply at the moment when the strength of current is close to zero (see A. V. Donskoy, V. S. Klubnikin, Elektroplazmennye protsessy i ustanovki v mashinostroyenii, Leningrad, "Mashinostroyenie", Leningradskoye otdelenie, 1979, p.99). It is obvious that the presence of additional power supplies results in the design complication and increases the cost of the plasma-arc furnace.
Also known in the art are furnaces of another type, wherein an attempt is made to increase the stability of a single-phase AC arc burning by using a plasma jet obtained as a result of heating a plasma-forming gas being fed to the torch by a pilot DC arc which is ignited between the electrode and the torch nozzle (see G. A. Farnasov, A. G. Fridman, V. N. Karinsky, Plazmennaya plavka, Moscow, "Metallurgiya", 1968, p. 83, FIG. 41, and M. F. Zhukov, V. Ya. Smolkov, B. A. Uryukov, Elektrodugovye nagrevateli taka (plazmotrony), Moscow, "Nauka", 1973, p. 40-41). In order to generate a pilot arc, an additional power supply is connected to the electrode and the nozzle of the plasma torch, namely a DC power supply. The plasma jet provides for constant ionization of the interelectrode space (between the torch electrode and the metal being heated), thereby promoting an increase in the stability of main arc burning.
The presence of an additional power supply in such furnaces, however, results in significant complication of their design. Besides, in the furnaces of the both above-mentioned types stabilization of arc burning can be achieved only with a relatively small arc length (not more than 200 mm) since for an arc of a greater length the furnaces of the first type will require complicated, expensive and dangerous in operation high-voltage power supplies, while in the furnaces of the second type stabilization by means of a plasma jet is practically impossible because of a low efficiency of the plasma jet. The low efficiency is caused, in particular, by high losses in the thermal energy which is removed together with water cooling the torch nozzle, and is dissipated by means of radiation and convection into the surrounding space.
More effective is a plasma-arc furnace with a three-phase connection of plasma torches (U.S. Pat. No. 3,147,329). This furnace comprises a casing with plasma torches mounted therewithin in a symmetrical relationship relative to the vertical axis thereof, each of said torches having a body provided with a nozzle and an axial conduit for feeding plasma-forming gas, and a cylindrical electrode disposed within said conduit for connection to a three-phase power supply. Moreover, in the given furnace, similar to those above described, there is provided a DC power supply for striking pilot arcs.
Three-phase connection of the plasma torches makes it possible to eliminate the hearth electrode and to further reduce the tendency of the torches to double arc formation. Such a connection also allows the power supplied to the metal being heated to be increased, thereby upgrading the efficiency and intensifying the technological process.
At the same time, in the operation of the above furnace there arise some serious difficulties. It should be noted first of all that the pilot arc ionizes the plasma-forming gas only within a relatively small portion of the main arc near the electrode of the plasma torch at the moment when the strength of current of the main arc is close to zero. The major portion of the gas column of the main arc within the space between the torch nozzle and the metal being heated is not ionized by the pilot arc since the power and the efficiency of the plasma jet generated by the pilot arc are relatively small.
At the moment when the current and voltage of the main plasma arc (AC current arc) reach zero values, the renewal of the energy dissipated from the column of the main arc into surrounding space is stopped, thereby resulting in the development of deionization processes within the interelectrode space of this arc. Since the problem of optimum mutual disposition of torches in said furnace is not solved, the conductivity of the interelectrode space in the majority of cases is lowered to such an extent that the ignition of arc during the next half-cycle of the current flow becomes either very difficult or impossible.
It should be also noted that in the course of burning of a pilot arc between the electrode and the torch nozzle there occurs an intensive heating of an interlayer of cold gas at the nozzle walls, said interlayer acting as electro- and thermal insulation between the plasma flow and the walls of the nozzle conduit. This heating results in a sharp deterioration of electric insulating properties of said gas interlayer, thereby causing gradual damage of the nozzle in throwing the arc thereon, and leads to the deterioration of thermal insulating properties of said interlayer, which fact also promotes the damage of the nozzle due to intensive heating thereof. A double arc appears, said arc burning in one area between the electrode and the torch nozzle and in other area between the nozzle and the metal being heated due to which fact the nozzle is subjected to the effect of concentrated energy generated by the main and pilot arc.
Finally, it is to be noted that the need in pilot arcs results in complication of the power supply circuit and the circuit for controlling the torches, and requires intensification of cooling the nozzles of these torches.
All the above difficulties inhibit wide utilization of the above described plasma-arc furnace.